Hello @jessarin814, welcome to the QuickSight Community!
Depending on what other values are being returned in your visuals, I think it is likely we can find a solution that works.
In regards to pivot tables, one of your options is to click the dropdown on the field that you want to target once it is added to your visual and select Format. You can then click More formatting options and the last option for Null values in the format data display pane. There you can click Show custom and put in whatever value you would like to display rather than null.
As for KPIs, and ifelse conditional where you return your field with an integer value or 0 should prevent No Data from showing.
Now if your grouping metrics are displaying NULL and your values, you may experience some issues. I will mark this as a solution for now, but if you still need assistance after trying some of these updates, let me know. I will need some more details about the fields you are using and why/when they would be returning NULL values.